In a snowy area, a quantity of rock salt and/or gravel are often scattered over the roads, which, when combined with sand and stones on non-pavement roads, inevitably cause damage to the coatings of automobile bodies and causes serious concerns in the related field. That is, a coating formed by a conventional coating composition is liable to be damaged such as to expose the substrate metal by stone chipping, which becomes easily rusted, thereby resulting in an inferior appearance. In order to overcome these serious problems, there have been proposed various methods, i.e. a method of applying, between top and under coats, an intermediate coating containing ultrafine sericit particles (as Japanese Patent Pub. No. 43657/77); a method of using an inter coat comprising an acidic resin, talc and an antirust pigment (as Japanese Patent Application Kokai No. 56165/80) and a method of applying a first intercoat containing a talc and then a second intercoat containing no talc, in a wet on wet system (as Japanese Patent Publication No. 45813/78.
However, these methods are all based on the idea that when the coating is chipped, the inter coat will be peeled off, thereby absorbing said chipping impact and preventing the damage to the metal plate substrate. This chipping, however, makes for a poor film appearance and the protection performance of the coating is diminished.
Furthermore, an increase in the number of coating operations is clearly undesirable from the viewpoint of manufacturing efficiency.
In a coating system involving an electrodeposition-inter coating-top coating as used in the automobile industry or the like, one cannot but rely on hard multi-layer coatings from the standpoint of generic performance, weather resistance, appearance of the coating and the like. However, when the coating is chipped, the impact strength is transfered to the steel substrate and hence the substrate is easily damaged and rusted. Therefore, at the under floor and side sill portions of automobile bodies, it is a general practice to apply a coating containing, as a main ingredient, a very soft material like a vinyl chloride sol, a rubber latex, a urethane resin and the like in a film thickness in the order of 200 to 500.mu.. Such coating is indeed excellent in chipping resistance because of its higher elongation properties, but has the drawback of an orange peel-like and inferior appearance an, inferior general generic performance due to deficient tensile strength and softness, and poor weather resistance. Therefore, such coatings cannot be applied over the surfaces of the outer plates of automobile bodies which extend above the side step thereof. Thus, in the coating of automobile bodies, hard and soft paints are separately applied on the upper and the lower portions of the borderline of the side step. It is needless to say, however, that even at the under floor portion, preference is given to a coating which exhibits excellent coating performance, weather resistance, and has a good appearance.
In general, it has been well known that the softer the film softness, the larger the elongation and the lesser the tensile strength of the film.
According to the inventors' studies, it has been found that if the film has a larger elongation, then the substrate damage and hence rust generation hardly occurs when the coating is chipped with stones, but there are such disadvantages that cohesion failure of the film is liable to occur because of its inferior tensile strength, thereby causing interface failure and exposure of the substrate surface. This results in the deterioration of the film appearance. The inventors have further continued the studies and have succeeded in finding a coating composition having well balanced properties with respect to each of these conflicting properties.